The annual celestial events that occur from our perspective on Earth have long been studied and observed by civilizations throughout time as having various meanings. An Equinox is an event that occurs twice each year. The first time is around March 20th and the second time is near September 22nd.
These two events have been tied, along with the solstices, to the seasons. They are also celebrated in various cultures and civilizations for various reasons. Join us as we examine the science behind these bi-annual events and how they are celebrated in cultures around the world and throughout time.
The term for this event is derived from the Latin word aequus meaning “equal” and nox, meaning “night:” The word has had several different meaning throughout history, but the oldest definition is a day in which daytime and night are approximately equal in length.
This definition isn’t entirely accurate because of several reasons: the first of which being that sunrise is defined as the point when the sun begins rising above the horizon. This time is different depending on where the location is on the earth in regards to latitude and longitude. The Earth’s atmosphere also refracts sunlight which results in there being daylight before the sun has begun to rise.
The other definition of the term refers to an astronomical event that occurs on two specific days throughout the year. This event is when the plane of Earth’s equator passes across the center of the Sun. At this point, the natural tilt of Earth’s axis is equal with the sun and not tilting away from, or toward it.
During these two times each year, the sun is exactly overhead, known as the subsolar point, at Earth’s equator. The Sun is also at zenith over the equator. During the March event the Sun moves northward from the equator, and in September it moves south.
During one of these events, the Sun is located at one of two points on opposite sides of the celestial sphere. The celestial equator and ecliptic intersect in this moment at one of two equinoctial points. One is known as the vernal point for the March event, and the other is known as the autumnal point for the September event.
Ultimately, what this means is that during the event, both the northern and southern hemispheres of Earth are equally illuminated by the Sun. On this day, the center of the Sun maintains an approximately equal amount of time above and below the horizon on every location across the surface of the Earth.
As a result, day and night are about the same length. Truthfully, the day is still slightly longer than the night, but the gap is never closer than on one of these days. Here are some final factors that contribute to the fact that the length of day and night are never truly equal across the Earth:
Julius Caesar created a calendar in 45 BC that set March 25th as the spring event of equal day and night. The Julian year (365.25 days) is actually longer than the actual length, and this resulted in the date drifting each year. The event occurred on March 21st in 300 AD and by 1500 AD the date had moved to March 11th.
As a reaction to this apparent drift in the date, Pope Gregory XIII created what we know as the modern Gregorian calendar. The pope’s motivation was in regards to the date of Easter set forth by the Council of Nicaea in 325 AD. He wanted to keep the date of Easter in conformity with their ruling, so he moved the date of the March event to March 21st.
Of course, the leap year once again meddled with the date, causing it to shift up to 53 hours around the established date. This created the possibility where the event would fall on March 22nd, thus making it occur after Easter.
A number of days were removed from the calendar to ensure that the event would always fall between the 19th and 21st of March. Below are the various names associated with the event throughout history:
The Persian Calendar starts each year on the day of the March event, which is determined at Tehran. The Indian National Calendar also begins their year on the day next to the vernal event on March 22nd or March 21st in leap years.
The Julian calendar reforms added length to seven different months and added a day to February every four years for leap year. The calendar was based on each year being 365 days and 6 hours long. The discrepancy between this length and the actual length of a year around the tropics resulted in the vernal event falling on March 10th or 11th in this calendar.
The day of Jewish Passover falls on the first full moon after the Northern Hemisphere’s vernal event each year. In the Christian set of traditions, Easter is tied to the date of the vernal event as well. It occurs on the first Sunday after the full moon that occurs on or after the event.
Eastern Orthodox Churches use the older Julian calendar, while the Western Churches use the Gregorian calendar. This results in the actual date of Easter being different between the two churches. The earliest possible date for either version of the calendar for Easter is March 22nd, and the latest date is April 25th.
In West Asia, the vernal event is the first day for a number of different calendars. The Iranian calendar for example celebrates their new year’s festival of Nowruz on March 20th or 21st. This festival is based on ancient Persian mythology which tells of a king named Jamshid taking the throne on that day.
The festival lasts two weeks each year and celebrates the Persian creation story. The same festival is also celebrated in other countries:
In addition, many Arab countries celebrate Mother’s Day on the vernal event in March each year. In Africa, an ancient Egyptian holiday called Sham ennisim is celebrated each year as a national holiday meant to welcome the beginning of spring on this the vernal event.
When Egypt was converted to Christianity the date was moved to Easter Monday, but before this it was always on the day of the vernal event. In today’s modern society, this event in March is also celebrated in a number of ways:
As you can see, the March or vernal equinox is celebrated by numerous cultures and people as a day to welcome the beginning of spring in the northern hemisphere, or the beginning of winter in the southern hemisphere where it is known as the autumnal or fall event. Before we finish, let us also examine the celebrations for the coming of winter and the second annual event.
For the French Republican Calendar, the September event marks the first day of the year. Google also celebrated this event in 2014 with its own “Google Doodle.” This animation depicted a person walking through a forest where the leaves changed color as he walked past.
In the Iranian Calendar the September event is the first day of the Mehr or Libra in a festival called Jashne Mihragan. This same day is known as the festival of sharing or love in Zoroastrianism.
In Korea an annual harvest festival known as Chuseok is celebrated for three days around the Autumn event. In China, the Mid-Autumn Festival is celebrated near the September event. The actual date is the 15th day of the 8th lunar month. It is an official holiday in China, Hong Kong, Taiwan, and other countries in eastern Asia. Japan has a holiday named after the event each year.
In Europe, the Romans celebrated on this September day each year in regards to the goddess of fruits and growing, Pomona. Modern Pagans, known as Neopagans, celebrate the fall event as part of the “Wheel of the Year.” This wheel is built around the seasons of the year and can consist of either four or eight festivals.
When referring to the seasons, these two terms are used often. Both of them are in reference to the Sun’s path throughout a year on Earth, but they have different meanings. As we know, the latter term refers to the two days each year when the Sun crosses the equator, resulting in roughly equal lengths of day and night.
The solstices refer to two separate days when the sun is at its highest and lowest point. The first point is known as the Tropic of Cancer and the lower point is known as the Tropic of Capricorn. The summer solstice occurs each year around June 21st and the winter solstice occurs around December 22nd. The name refers to the Latin word “solstitium” which means “Sun standing still.”
The celestial events that happen each year hold different meanings for various cultures and people. For astronomers it’s another opportunity to understand, at least a little bit more, how our universe works.